1. Field of the Invention
The present invention relates to a divider/combiner and more particularly to a divider/combiner for dividing or combining radio frequency power applicable to communication units utilizing a microwave band or the like.
2. Description of the Related Art
In recent years, various kinds of communication units like cellular phone units have remarkably been popularized. Reflecting such tendencies, it has become more and more necessary to further improve the performance of power dividers/combiners operating in a microwave band and power amplifiers using various kinds of semiconductor devices such as transistors and field effect transistors (FETs). In addition, researches have also been vigorously carried on for developing such units operating in a higher frequency band (i.e., a so-called "millimeter wave band"). Thanks to such rapid development, some of such units have already been used practically.
A Wilkinson type radio frequency (hereinafter abbreviated as "RF") power divider/combiner is known as a representative conventional RF power divider/combiner. FIG. 1 is an equivalent circuit diagram of a conventional Wilkinson type RF power divider/combiner. When the divider/combiner shown in FIG. 1 is used as a divider, a node 101 receives RF power and nodes 102 and 103 output the received RF power. On the other hand, when the divider/combiner shown in FIG. 1 is used as a combiner, the nodes 102 and 103 receive the RF power and the node 101 outputs the received RF power.
Herein, the impedance of a circuit connected to the node 101 will be denoted by Z1 while the impedance of circuits connected to the nodes 102 and 103 will be denoted by Z23. By setting the characteristic impedances of transmission lines 111 and 112 to be about .sqroot.(2.multidot.Z1.multidot.Z23) and the lengths of the transmission lines 111 and 112 to be about .lambda./4, an impedance matching is realized between the divider/combiner and an external circuit connected to the divider/combiner. By providing a resistor 121 having a resistance of about 2.multidot.Z23 between the nodes 102 and 103, an electrical isolation is realized (in this specification, the expression "an isolation is realized" will always mean that an electrical isolation is realized) between the nodes 102 and 103.
The isolation between the nodes 102 and 103 is realized because the RF power passing through two paths is cancelled by each other, as will be described later. Suppose the RF power is input through the node 102 and output through the node 103. Then, one path is formed by the node 102, the transmission lines 111 and 112 and the node 103, while the other path is formed by the node 102, the resistor 121 and the node 103. Since the lengths of the transmission lines 111 and 112 are about .lambda./4, a phase difference between the RF power passed through one of these two paths and the RF power passed through the other path becomes about .pi. rad at the node 103 (i.e., the phase difference is about 180.degree.). Thus, by adjusting the resistance of the resistor 121, the RF power (input through the node 102 and passed through these two paths is cancelled at the node 103. In other words, the nodes 102 and 103 are electrically isolated from each other with respect to the RF power.
However, in accordance with conventional technologies, only a case where an impedance having a real component (i.e., a resistance component) is connected to each of the nodes 101, 102 and 103 has been considered.
On the other hand, a power amplifier described in Japanese Laid-Open Patent Publication No. 7-263981 provides a specific measure for suppressing a parasitic oscillation in a power amplifier using a power divider/combiner. In this patent publication, it is described that a gain is decreased by inserting a resistor element and a phase delay element, which are serially connected to each other, in between the two transmission lines of the power divider, thereby preventing the oscillation.
The phase delay element described in this patent publication is asymmetrically disposed with respect to the resistor element. In such an arrangement, since the phases of the RF power input to the resistor element are not the same, the resistor element consumes the power. The power amplifier described in this patent publication has a problem in that the output power and the gain of the power amplifier are disadvantageously decreased because of the power lost by the resistor element.
Furthermore, it is generally critical for an amplifier using a power divider/combiner to realize an isolation between the nodes. However, the power amplifier described in this patent publication cannot surely realize the isolation between the nodes.
FIG. 2 is an equivalent circuit diagram of a conventional Wilkinson type power divider, in which one input signal is divided into three and then output through three nodes (i.e., the division number is three). The Wilkinson type power divider shown in FIG. 2 includes: nodes 201, 202, 203 and 204; transmission lines 211, 212 and 213; and resistors 221 and 222. In order to simultaneously realize the impedance matching and the isolation, the lengths of the transmission lines 211, 212 and 213 must be set at about .lambda./4. Thus, the transmission line 213 must be laid out in a meander line shape. However, as the division number becomes larger, it becomes more and more difficult to lay out the transmission lines so that all of these lines have an equal length.
In the conventional Wilkinson type power divider/combiner shown in FIG. 1, as the operating frequency becomes higher, the distance between the nodes 102 and 103 becomes more and more critical so that the characteristics of the isolation between the nodes 102 and 103 are adversely deteriorated. To sum up, conventional RF power dividers/combiners have the following problems.
(1) In conventional RF power dividers/combiners, it has been difficult to simultaneously realize an impedance matching and an isolation if an impedance having an imaginary component (i.e., a reactance component) is connected to either an input node or an output node. That is to say, it is a novel idea conceptualized by the present inventors for the first time that an impedance matching and an isolation are simultaneously realized even if an impedance having a reactance component is connected to either an input node or an output node, as will be described later.
(2) In conventional power amplifiers using RF power dividers/combiners, it is difficult to realize an impedance matching and an isolation between the gate terminals thereof and similar problems are also caused between the drain terminals thereof, because the input/output impedance of a FET ordinarily has an imaginary component.
(3) In conventional power amplifiers using RF power dividers/combiners, an equal potential is provided to the gates and the nodes of a plurality of FETs. Thus, when the FETs have different threshold voltages, the output waveform becomes adversely asymmetric so that the RF power division/combination efficiency is deteriorated.
(4) In conventional RF power dividers/combiners using transmission lines, as the division (or combination) number becomes larger, it becomes more and more difficult to equalize the lengths of all the lines. Thus, when the lengths of the lines become different, the impedances cannot be matched any longer and the isolation cannot be realized satisfactorily.
(5) In conventional RF power dividers, in the case where the distance between the output nodes cannot be neglected with respect to the wavelength, the isolation between the output nodes cannot be realized satisfactorily.